1. Field of the Invention
This invention relates to methods of and apparatus for transmitting signals over the lines of an alternating current power distribution network using audio-frequency signals.
2. Description of the Prior Art
The best known method of transmitting signals over the lines of an alternating current power supply network is the audio-frequency power line carrier control method, otherwise known as the ripple control method. In this method, audio-frequency signals of small band-width are transmitted from one or a few centrally located transmitters over the power supply network, and are evaluated in a plurality of receivers distributed about the network, for carrying out instructions of different types. In this method, the transmission speed and consequently the amount of information transmitted per unit of time is relatively small.
Methods are also known which, in many respects, are similar to that described immediately above, and in which for example the counter state of counters distributed about the network, or information concerning the carrying out of instructions in the audio-frequency power line carrier control method are reported back to a central station. Throughout this specification instructions and information will be referred to collectively as "intelligence". Reporting takes place in the opposite direction to the flow of power, and for reporting back it is necessary to have a plurality of transmitters with a relatively small capability, and which must be inexpensive, whilst there is only one or a few centrally located receivers, which may therefore be relatively expensive. Thus, it has been proposed to report the consumption of fixed quantities of energy with the aid of devices actuated by audio-frequency power line carrier control signals, which cause the instantaneous coupling of a resonant circuit between two power lines in the individual counters, followed by the evaluation of the signals so transmitted to a control receiver having extremely selective electronic filters (Austrian Pat. No. 241,589). A later method has been described in which for reporting in the reverse direction to the flow of power in the network, extremely narrow band signals from a band between two power supply frequency harmonics, for example between 120 and 180 Hz is used in a frequency division multiplex method for transmitting information from a plurality of stations distributed about the network to a central station. Reporting back can be initiated by the normal audio-frequency power line carrier control system. The difficulty of reporting back over different network levels, involving passage of the signals through the intermediate transformers, is allegedly eliminated in a further proposal for reporting back counter states via the network by using relatively high audio-frequencies between 1 and 20 kHz and transmitters with a low resistance output and with a power level of at least 1 watt (German Offenlegungsschrift No. 2,445,388). Finally, Swiss Pat. No. 504,139 proposes for the purpose of increasing the quantity of information to be transmitted and the transmission speed, to limit the length of the pulses to be transmitted to a maximum of one eighth of a cycle of the power supply voltage, whilst the signals can have a spectrum of several hundred hertz corresponding to the transmission range of the network.